Admixture mapping is a novel approach to identifying genes for complex traits in populations of mixed ancestry. Admixture mapping works if the alleles which pre-dispose to disease, or to different subtypes of disease, have different frequencies in the ancestral populations which contributed to the admixed population. Latino/Hispanic populations are an admixed group with ancestry from European, Native American and African populations. Latinos in the San Francisco Bay Area are mostly of Mexican and Central American origin who are mostly of mixed European and Native American descent. We have recently identified a set of markers for admixture mapping in this population that are informative for European-Native American ancestry difference that provide whole genome coverage. We have also recently investigated the association between ancestry and breast cancer risk among Latinas in the San Francisco Bay Area. Our preliminary data suggests that among Latinas, those with higher Native American ancestry may be at increased risk of pre-menopausal breast cancer. However, among post-menopausal women, those with higher European ancestry appeared to be at higher risk of breast cancer. The interaction between pre- and post-menopausal breast cancer risk and ancestry remained significant after adjustment for known risk factors. Based on these findings, we hypothesize that some alleles which pre-dispose to breast cancer at different ages may be identifiable by an admixture mapping approach. We propose an admixture mapping study to identify genetic modifiers of breast cancer risk among Latina women in the context of an ongoing case-control study of breast cancer. The total population that will be available for analysis will include 1000 Latina cases and 1000 controls.